In recent years, higher integration, higher function, and highly dense packaging have been accelerated in semiconductors widely used in electronic apparatuses, communication apparatuses, and personal computers, and demands for properties of laminates for semiconductor plastic packages and high reliability thereof have been increased. Lead-free soldering is typically used because of increasing environmental concerns in particular. For this reason, laminates having high heat resistance suitable for a reflow step at high temperatures have been required.
A reduction in the coefficient of thermal expansion in the planar direction of the laminate has been strongly demanded in recent years for the following reason. When the difference in the coefficient of thermal expansion between a semiconductor element or a semiconductor plastic package and a laminate such as a printed circuit board for a semiconductor plastic package is great, a thermal impact applied to the laminate causes warpage in the laminate. This warpage causes connection deficits between the semiconductor element and the laminate or between the semiconductor plastic package and the laminate to be disposed.
Known methods of reducing the coefficient of thermal expansion in the planar direction of the laminate include a method comprising adding an inorganic filler to a resin composition.
Another known method comprises incorporating an organic filler having rubber elasticity into a varnish containing an epoxy resin (Patent Literatures 1 to 5).
Furthermore, a method comprising using silicone rubber as rubber elastic powder is known (Patent Literature 6).
Besides these, a method comprising incorporating a silicone resin, a siloxane modified resin, or an epoxy modified silicone resin into a varnish is known as another method of reducing the coefficient of thermal expansion.